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For millions of years the hominid brain has grown increasingly bigger. But for the last 10,000 years, the human brain has been shrinking at an alarming rate and no one really knows why.

The Ups and Downs of Brain Size

For the past 800,000 years, brain size has increased at a rate of about 7cc every 10,000 years. But in the past 10,000 years, brain size has decreased by 150cc. That’s over 200,000 years of brain growth lost in just 10,000 years.

The Ups and Downs of Body Mass

Body mass changed with the brain size at a rate of 1 kilogram of body mass for every 4.3cc of brain size. In the past 10,000 years, body mass has decreased by 5 kg. That’s 30,000 years of growth lost in just 10,000 years. This means our brains are shrinking faster than our bodies. If our brains shrank as slow as our bodies, we would have an extra tennis ball worth of brain matter. If our bodies shrank as fast as our brains, we would be about 4’ 6” tall and weigh 64 lb.

The Ups and Plateaus of EQ

Does this mean we are dumber?

Scientists have found a close relationship between intelligence and the encephalization quotient, or EQ. It is the ratio between actual brain mass and predicted brain mass for an animal of a given size. The formula for mammals looks like this: EQ=E/0.12 P2/3 , where E = brain mass, and P = body mass. Throughout hominid history, EQ has consistently risen. But for the past 20,000 years, eq has remained the same.

So What’s the Big Idea?

No one really knows for sure why our brains are shrinking but following are some proposed ideas.…


Around 800,000 years ago the earth started experiencing climate fluctuation which coincided with fast brain growth. Cold weather is survived by bigger bodies and therefore bigger brains. Warming trends in the past 20,000 years have favored smaller bodies and therefore smaller brains.


The advent of agriculture led to unhealthy grain-heavy diets (lacking protein and vitamins). Body sizes and brain sizes responded. Those with more energy hungry grey matter in their heads would die off, lacking nutrition.

Social Complexity

As complex societies emerged, those with smaller brains could survive with the help of others. A higher survival rate allowed smaller brains to populate the gene pool. Increased population density leads to increased division of labor. When population is sparse, brains grow because you need to know more to survive. With the division of labor you do not have to know as much. Mistakes in judgment are less likely to be fatal in more supportive societies.


The brain constitutes 2% of the human body, but it uses 20% of the body’s resources. The larger the brain, the more fuel it takes to formulate thoughts. As gene pools grow, the most efficient populations excel.


As violence and aggression is bred out of domesticated animals, they lost brain mass. Animals that remain juvenile longer are easier to domesticate (like humans). Bonobos have brains 20% smaller than chimps. They act like juvenile Chimpanzees, and consequently function as domesticated chimps.

Common characteristics of domesticated animals include:

• Smaller teeth

• Flatter faces

• More striking range of coloration and hair types

The History of Brain Sizes

Sahelanthropus tchadensis
Lived: 7 – 6 (6.5 mean) mya
Brain Size: 282cc – 500cc (350cc mean)

Ardipithecus ramidus
Lived: 4.35 – 4.45 (4.4) mya
Brain Size: 300cc – 350cc (325cc)

Australopithecus afarensis
Lived: 3.85 – 2.95 (3.11 mean) mya
Brain Size: 387cc – 550cc (445.8cc mean)
Brain Weight: 435g
EQ: 2.2
Body Weight: 42 kg
Body Height: 151 cm

Australopithecus africanus
Lived: 3.3 – 2.1 (2.7 mean) mya
Brain Size: 400cc – 560cc (461.2cc mean)
Brain Weight: 450g
EQ: 2.5
Body Weight: 41 kg
Body Height: 138 cm

Paranthropus aethiopicus
Lived: 2.7 – 2.3 (2.1 mean) mya
Brain Size: 400cc – 490cc (431.8cc mean)
EQ: 3.4
Body Weight: 38 kg
Australopithecus garhi
Lived: 2.5 mya
Brain Size: 450cc
Brain Weight: 445g

Homo habilis
Lived: 2.4 – 1.4 (1.8 mean) mya
Brain Size: 509cc – 687cc (609cc mean)
Body Weight: 32 kg
Body Height: 100 – 135 cm
Paranthropus boisei
Lived: 2.3 – 1.2 (1.7 mean) mya
Brain Size: 475cc – 545cc (508.3cc mean)
Brain Weight: 515g
EQ: 2.7
Body Weight: 34 – 49 kg
Body Height: 124 – 137 cm

Australopithecus sediba
Lived: 1.977 – 1.98 (1.9785 mean) mya
Brain Size: 420cc – 450cc (435cc mean)
Homo rudolfensis
Lived: 1.9 – 1.8 (1.865 mean) mya
Brain Size: 752cc – 825cc (788.5cc mean)
Brain Weight: 735g
EQ: 5.1
Body Weight: 46 kg

Homo erectus
Lived: 1.89 – 0.14 (.72 mean) mya
Brain Size: 727cc – 1390cc (990cc mean)
EQ: 5
Body Weight: 40 – 68 kg
Body Height: 145 – 185 cm

Homo ergaster
Lived: 1.8 – 1.3 (1.7 mean) mya
Brain Size: 750cc – 848cc (800.7cc mean) 850g
EQ: 4.5
Body Weight: 58 kg
Paranthropus robustus
Lived: 1.8 to 1.2 (1.5 mean) mya
Brain Size: 450cc – 530cc (493.3cc mean)
Brain Weight: 525g
EQ: 3
Body Weight: 40 – 54 kg
Body Height: 100 – 120 cm

H. e. georgicus
Lived: 1.7 mya
Brain Size: 650cc – 780cc (715cc mean)

Homo antecessor
Lived: 1.2 – 0.8 (1 mean) mya
Brain Size: 1,000cc – 1,150cc (1075cc mean)

Homo e. soloensis
Lived: 0.55 – 0.143 (.347 mean) mya
Brain Size: 1013cc – 1251cc (1144.6cc mean)

Homo heidelbergensis
Lived: 0.7 – 0.2 (0.339 mean) mya
Brain Size: 1165cc – 1450cc (1268cc mean)
EQ: 5.3
Body Weight: 51 – 62kg
Body Height: 157 – 175cm

Homo s. neanderthalensis
Lived: 0.2 – 0.028 (0.081 mean) mya
Brain Size: 1172cc – 1740cc (1420cc mean)
EQ: 5.5
Body Weight: 65 kg
Body Height: 164 cm

Homo s. sapiens
Lived: 0.2 – present (0.044 mean) mya
Brain Size: 1090cc – 1775cc (1457cc mean)
EQ: 7
Body Weight: 64 kg

Homo s. idaltu
Lived: 0.16 mya
Brain Size: 1450cc
Homo floresiensis
Lived: 0.095 – 0.013 (0.054 mean) mya
Brain Size: 426cc
EQ: 3.6
Body Weight: 30 kg
Body Height: 106 cm

H.s. erectus “Cro Magnon”
Lived: 0.03 – 0.02 (0.025 mean) mya
Brain Size: 1590cc – 1730 (1660cc mean)
Body Height: 166 – 171cm

Modern Human
Brain Size: 975cc – 1499cc (1350cc mean)
EQ: 7.44
Body Weight: 58kg
Body Height: 165 – 175cm